The present invention relates to a method of preparing highly reactive nickel catalysts with an increased lifespan that are used in preparing synthesis gas by reforming methane, a main component in natural gas, with carbon dioxide. More particularly, the present invention relates to a method of preparing nickel-alumina hybrid aerogel catalyst by using a sol-gel method or by having alumina aerogel to carry nickel thereon, in a different manner than the prior nickel catalyst supported on alumina.
Generally, an aerogel is prepared by supercritically drying a gel from a sol-gel reaction. The aerogel has a specific high surface area ratio with a low density. Due to its physical characteristics, aerogel is useful as a catalyst or can be used as a carrier for a catalyst. Supercritical drying is a process of drying the gel above a critical state such as high temperature and high pressure to remove solvents held in the gel structures. By this method, it is possible to prevent shrinkage and crack formation due to capillary pressure differences between the gas/liquid phase interface.
Carbon dioxide is considered to be relatively harmless. However, it has been recently revealed as a major factor in global warming, and its production is now restricted by climate control regulation. Therefore, people are now more interested in reducing carbon dioxide production or in developing physical and chemical methods to fix carbon dioxide.
One of the chemical methods used in fixing carbon dioxide is to produce a mixture containing carbon monoxide and hydrogen by reforming methane, a major component in natural gas, with carbon dioxide as shown below. The produced mixture can be utilized in generating a variety of chemicals. This is one of the more effective ways of utilizing the natural gas, which is used mainly as a fuel. It is practically useful to use methane and carbon dioxide simultaneously, considering that methane has a higher global-warming tendency than carbon dioxide and when it is inconvenient to transport the methane, it is wasted by burning it in-situ, which produces undesirable carbon dioxide.
CH4+CO2xe2x86x922CO+2H2(xcex94H=261 kJ/mol)
The synthesis gas or xe2x80x9csyngasxe2x80x9d produced by the reaction above has a carbon monoxide/hydrogen ration of 1:1 and can be used as a starting material for many chemicals. The purity of the produced carbon monoxide is known to be higher than that produced by any other methods. This reaction characteristic is similar to the conversion method using steam in producing hydrogen gas, and therefore already established processes and knowledge in the field can be utilized. A durable catalyst is urgently needed to be developed since catalyst can be easily deactivated due to coking during the reaction when the commercial nickel catalyst for the steam reforming is used.
According to the literature, the deactivation of the noble metal catalyst is markedly slow (A. T. Ashcroft, A. K. Cheetham, M. L. H. Green, and P. D. F. Vernon, Nature, 225, 352 (1991)). Considering the economical aspect, however, it is impractical to use the noble metal catalysts when a large amount of catalyst is needed. Therefore, there have been many studies to improve the durability of the nickel catalyst by using a co-catalyst or additives.
In the commercialized SPARG process, nickel catalyst treated with sulfur is used to improve the durability. In this process, however, there are many problems such as the sulfur must be supplied continuously to the reaction mixture and the reaction needs to be carried out at a temperature higher than 900xc2x0 C. to improve the reactivity of the catalyst [H. C. Dibbern, P. Olesen, J. R. Rostrup-Nielsen, P. B. Tottrup, and N. R. Udengaard, Hydrocarbon Process., 65, 71 (1986)]. The deactivation of the catalyst can be reduced by adding vanadium or molybdenum into the nickel catalyst [T. Arakawa and M. Oka, U.S. Pat. No. 3,849,087], by adding a basic material such as calcium oxide [Z. L. Zhang and X. E. Verykios, Catal. Today 21, 589 (1994)] or by using basic magnesium oxide that can contain the catalysts.
It is an object of the present invention to provide a durable catalyst free from deactivation problems due to coking. The catalyst of the present invention is useful in the process of simultaneous treatment of methane and carbon dioxide which are major contributors of global-warming and is different from a prior nickel catalyst for steam reforming.
More particularly, it is an object of the present invention to provide a highly reactive nickel catalyst with an increased lifespan that is used in preparing a synthesis gas by reforming methane, a main component in natural gas, with carbon dioxide. That is, the present invention relates to a method of preparing nickel-alumina hybrid aerogel catalyst by using a sol-gel method or by having alumina aerogel to carry nickel thereon, in a different manner than the prior nickel catalyst supported on alumina.
One of the simplest processes for preparing alumina gel is the hydrolysis of aluminum alkoxide using an acid catalyst and excess amount of water. In this case, it is required to exchange water with another solvent such as alcohol to prepare the aerogel by supercritical drying [B. E. Yoldas, J. Mater. Sci., 10, 1856 (1975)]. In the case of using an organic solvent such as benzene or alcohol, it takes several days to synthesize gels even at high temperatures, and therefore, an additive to accelerate the sol-gel reaction is often used [Y. Mizushima and M. Hori, J. Non Cryst. Solids, 167, 1 (1994)]. However, in the sol-gel method of the present invention, a homogeneous alumina gel can be synthesized at room temperatures without additives by slightly altering the solubilization, hydrolysis and condensation processes.
A detailed description of the preparation method of the alumina or the nickel-alumina hybrid aerogel by using the sol-gel method follows.
The preparation method of the alumina or the nickel-alumina hybrid aerogel of the present invention is composed of: the step of dissolving aluminum alkoxide in a heated alcoholic solvent; the step of forming a transparent sol by partial hydrolysis; the step of forming a gel at room temperature; the step of aging the gel; the step of supercritically drying the gel; and the step of thermally treating the gel.
The first step of dissolving aluminum alkoxide involves adding aluminum alkoxide into a heated alcoholic solvent and vigorously stirring it for dissolution. The aluminum alkoxides represented by Al(OR)3 (R is an alkyl with 1 to 6 carbons), preferably aluminum isopropoxide or aluminum sec-butoxide, can be used for this process. Any kind of alcohol, preferably ethanol or 1-propanol, can be used. The ratio between the alcohol and alkoxide is preferably 0.4 to 0.6 mmol/ml, and the temperature of the heated alcohol is preferably 40 to 90xc2x0 C.
The second step of forming a transparent sol by a partial hydrolysis involves adding the heated alcohol solution with aluminum alkoxide into a mixture of acid, water and alcohol in a single step, and stirring while heating the solution continuously to form a transparent sol by partial hydrolysis. There is no limit to the amount of alcohol used for this step, however, it is preferable to use ⅓ to ⅔ of the amount that was used for the first step. It is also preferable to use water in a mole ratio of 0.2 to 0.8 based on aluminum. It is preferable to use acid in a mole ratio of 0.01 to 1.0 based on aluminum. For instance, in the case of nitric acid, it is preferable to use the nitric acid in the amount of {fraction (1/50)} to {fraction (1/100)} in a molar ratio based on aluminum.
In the case of forming a nickel-alumina hybrid gel, completely dissolved alcohol solution containing nickel as a salt form at a 0.03 to 0.3 mole ratio to the total amount of aluminum is added. There is no limit to the amount of alcohol used for this step, however, it is preferable to use 0.05 to 0.2 of the amount that was used for the step of forming the transparent gel by partial hydrolysis. Any nickel salt that dissolves well in alcohol, preferably nickel nitrate or nickel acetate can be used.
The third step of forming a gel at room temperature involves cooling the transparent sol prepared in the second step and of adding an alcohol that contains a certain amount of water to form a transparent homogeneous gel in a few minutes. It is preferable to use water in the mole ratio of 0.5xcx9c1.5 based on aluminum.
The fourth step involves aging the gel formed at the third step for more than 1 hour.
The fifth step of supercritical drying the gel involves incubating the aged gel at a higher than the critical pressure and temperature of the alcoholic solvent and changing the condition to room temperature at atmospheric pressure. Supercritical drying can also be performed by replacing the solvent with carbon dioxide and by supplying carbon dioxide at the supercritical condition as above to completely remove the alcohol to form an aerogel.
The sixth step of thermal treating the gel involves initially thermal treating the gel at an inert atmosphere at 200 to 500xc2x0 C. and a secondary thermal treating in air or oxygen atmosphere at higher than 500xc2x0 C. The thermally treated alumina aerogel can be used as a support for carrying the nickel. The nickel-alumina hybrid gel, which went through the second step of adding the transparent sol into the alcohol solution which contains the completely dissolved nickel salt and through the third through the sixth step, can be used as a catalyst for the reforming reaction of methane with carbon dioxide. The increasing temperature rate is 1 to 20xc2x0 C./min for all the processes.
As described above, a synthesis gas is produced by using the catalysts prepared in the present invention. In this case, by using a conventional fixed-bed atmospheric flow reactor, first the hydrogen is supplied into the catalyst bed, then the hydrogen supply is terminated and nitrogen is added to purge the hydrogen. The methane and carbon dioxide reactants are supplied with nitrogen continuously to produce the synthesis gas. The gas contact time in the catalyst bed during the pretreatment and reaction is 0.0005 to 0.005 g.min.1xe2x88x921, and the composition of the reactant is 15 to 30% methane, 15 to 30% carbon dioxide and nitrogen balanced to 100%.